An apparatus and process for forming products from thermoplastic polymeric material having three-dimensional patterns and surface textures is disclosed in U.S. Pat. Nos. 4,128,369 and 4,290,248, both of which are hereby incorporated by reference.
In the apparatus and process disclosed in said patents a thermoplastic material to be formed is heated above its glass transition temperature before introduction between travelling flexible belt molds, which revolve in opposed relationship. The flexible belt molds each include a thin, flexible sheet-metal belt of relative high thermal conductivity and form a traveling mold channel, at least one having a flexible three-dimensional pattern formed on its front face. Opposed nip rolls press the revolving belt molds against the entering thermoplastic material. At least one belt mold travels partially around the nip roll and impresses its three-dimensional pattern into the heated plastic material in a progressive localized rolling, squeezing action in the nip region. Thereafter, a series of backup rolls along the mold channel hold the traveling belt molds against the impressed material for maintaining the impression while being cooled by liquid coolant into the memory-retention state. A cooling liquid, mainly water at room temperature, is moved along the backup rolls and applied to the back surface (inside surface) of each thermal conductive steel belt for cooling each belt mold. After the plastic material has been sufficiently cooled to retain three-dimensional patterns, the flexible belt molds are separated from it. Large area architectural panels can be produced. Belt molds are shown as including wide, thin, endless, flexible metal belts, at least two-feet wide, having a wide flexible mold formed of a heat-resistant material, such as rubber, bonded to the metal belt.
The prior disclosed apparatus utilizes thin steel belts which revolve upon large flat surface drive pulleys. Steel belts suffer from the inherent problems of being susceptible to dents, crimped edges, and rust. They also require a weld seam. Steel belts present difficulties in the maintenance of alignment as they travel over flat surfaced metal pulleys.
Steel belts are susceptible to denting, crimping, rusting, and camber because they are extremely thin, being typically 0.025 to 0.075 of an inch thick. The use of such extremely thin steel belts in an industrial environment increases the probability of incurring damage thereto.
The use of a metal belt requires that the drive pulleys be comparatively large because the metal belt cannot be made to continuously travel over small-diameter pulleys. Small pulleys cause bend-yield-stress elongation of thin metal belts. Thus, the drive pulleys in the prior disclosed apparatus must be fabricated of a sufficient diameter to accommodate the metal belt and consequently the drive pulleys occupy a substantial amount of space within the disclosed machine. Thus, the space available for cooling and other apparatus is strictly limited.
A weld seam is required in the formation of an endless-loop metal belt. An elongate planar sheet of metal is looped about itself and welded together to form the belt, thus forming a weld seam. The formation of such an endless-loop metal belt, without damaging it, is necessarily a time-consuming and somewhat difficult task. The weld seam should be made to keep the belt edges parallel to each other and be ground flush to prevent distortion of the flexible rubber mold which is to be subsequently formed on the outer surface of the welded planar metal belt.
Further difficulties in the maintenance and alignment of the metal belt of the apparatus disclosed in said patents occur because the metal belt is installed upon flat pulleys which lack any self-aligning characteristics.
Sixty or more small-diameter rollers function to maintain the two travelling mold surfaces of the prior art apparatus in close contact. The small-diameter steel rollers rotate continuously and are continuously exposed to the liquid coolant, which is comprised mainly of water. They ar subject to frequent malfunction and require periodic maintenance. Also, the use of such numerous small-diameter rollers does not facilitate maximum intimate contact of the opposing travelling mold surfaces because of the many gaps inherently formed between such rollers. The multiplicity of these steel rolls causes the travelling flexible molds to experience considerable fluctuations in contact pressures as they successively travel over roller-gap-roller-gap-roller, etc.
In the prior disclosed apparatus, an offset is formed between the inside edge of the exit rollers and the path of the molded product to help strip the belt molds from the molded product. That is, the circumference of each exit roller is not tangential to the plane of one surface of the molded product path, but rather the bottom roll is downward and the upper roll is upward away from the molded product path, in order to help separate the belt molds from the molded product. This offset reduces the support provided to the molded product, thereby requiring that the molded product be sufficiently cooled and rigid to resist deformation prior to passing between the exit rollers.
The prior disclosed apparatus utilizes hydraulic actuators to tension the mold belts, provide a compressive force to maintain contact of the two opposing mold belts, and to lift the upper mold assembly off of the lower mold assembly to facilitate maintenance and the changing of mold belts.
As is well known in the art, hydraulic actuators require the use of a motor, pump, various hoses and valves, and actuator cylinders. The hydraulic system must be maintained in a leak-free condition in order to function properly and prevent contamination of the molded product. Hydraulic systems constantly consume electrical energy when the apparatus is operative. That is, the hydraulic motor and pump must constantly be running in order to provide pressure to maintain and change position of the hydraulic actuators. The motor and hydraulic pump are inherently noisy and commonly located in close proximity to the apparatus. This makes the working environment of the apparatus extremely uncomfortable and contributes to an unsafe and unhealthy working environment.
The prior art discloses an apparatus and process that primarily removes the heat of the hot plastic by moving cold water along the small diameter backup rolls against the backside of the thin steel belts. This back-surface water cooling method proves inefficient because the heat of the plastic must first pass through the thick low thermal conductive silicone mold on at least one belt mold. The silicone mold material has a low thermal conductivity with a K factor of about 0.10 compared with the mild carbon steel which has a K factor of about 26.0. The K factor values for the materials are expressed in units of BTU per hour through a square foot per degree Fahrenheit of temperature difference per foot.
In addition to the difficulty of removing the heat of the plastic through the low thermal conductive silicone, this prior art back-surface method of cooling did not provide the means to control the temperature of the belt molds. It is desirable for good molding conditions to have the belt molds consistently at about the same temperature as the molds first contact the hot plastic each time they return to the entry end.
The back-side fluid cooling method also involves a water sump under the machine; a water cleaning system; water chillers or water cooling tower; and a water recirculating system. This equipment needs constant maintenance, causes high humidity in the work place, and increases the cost of operating the machine.
Therefore, the prior disclosed apparatus and process has a variety of deficiencies which detract from its effectiveness, efficiency, and marketability. In view of the shortcomings of the prior disclosed apparatus, it is desirable to provide an apparatus and process which does not utilize thin steel belts and consequently is not susceptible to crimps and dents; does not have weld seams; is not susceptible to rust and camber; and does not have difficulties in the maintenance of alignment as it travels over flat surfaced pulleys. It would be desirable to provide an apparatus with belt molds that can include a ridge and a gear or cog arrangement that will fit and mesh with a matching grooved drive roll sprocket as a means of maintaining mechanical alignment, both laterally and in the forward motion feeding direction. It would also be desirable to provide an apparatus which does not use a plurality of small-diameter rollers to maintain intimate contact of the upper and lower belt molds. These rolls require periodic maintenance as they are subject to the effects of wear due to friction and to exposure to the coolant water being applied to the steel belts.
Further, it would be desirable to provide an apparatus which uses small-diameter entrance and exit rollers to reduce the length of the mold belts required, increase the space available for cooling and other equipment, and reduce the size and cost of the machine as a whole.
Further, it would be desirable to provide an apparatus which does not require the use of hydraulic actuators and consequently would eliminate the need for a motor, pump, various hoses and valves, and actuator cylinders, as well as the requirements for maintaining these items in a leak-free state. It would also be desirable to provide an apparatus which operates quietly and does not constantly consume electrical energy.
Further, it would be desirable to provide an apparatus which does not utilize an offset between the exit rollers and the plane of the product path so that support is continuously provided to the molded product as it travels the length of the machine onto the product conveyors.
Further, it would be desirable to provide an improved means and apparatus to remove the heat from the surface of the silicone molds. Extracting the heat through the silicone and the steel belt backing with cold water moving against the thin steel backing is an inefficient exchange of heat and limits production rates. Further, it improves the molding operation if the heat of the belt as it first contacts the hot plastic is controlled. This can be accomplished by controlling the cooling of the belt molds through a series of dampers in the cold air ducts that provide the means to vary the temperature of the molds by varying the amount of chilled air blowing on the belt molds as well as the ability to control when the belt molds will be cooled on their return to the entry end of the machine.